The planographic printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a versatile thermal processless printing plate material, which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent and which can be treated in the same manner as in PS plates, has been required.
As a thermal processless printing plate material, there is Thermo-Lite produced by Agfa Co., Ltd.
In a thermal processless printing plate material, an image is formed according to a recording method employing a thermal laser emitting light with infrared to near infrared wavelengths. The thermal processless type printing plate material employing this recording method is divided into two, an ablation type printing plate material, and a phase change type printing plate material.
Examples of the ablation type printing plate material include those disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773. Examples of the phase change type printing plate material include those disclosed in for example, Japanese Patent O.P.I. Publication No. 11-240270. In the phase change type printing plate material, the hydrophilic layer contains hydrophobe precursor particles, which are not removed during printing, and the hydrophilic layer at exposed portions changes to be hydrophobic.
When an on-press development type planographic printing plate material as described above is used which does not require any special development before mounting on a printing press, an image formation layer at non-image portions of the printing plate material is transferred to printing paper sheets or to a dampening solution, whereby a printing plate is obtained. However, this process has mainly two problems to be solved.
One is that it is difficult to prevent lowering of durability of an image formation layer at image portions and lowering of printing durability, since it is necessary that the image formation layer have some degree of water affinity in order to remove rapidly an image formation layer at non-image portions from the printing plate.
A planographic printing plate material is required to have a plate inspection property for checking before printing whether or not a correct visible image is formed on a planographic printing plate material after imagewise exposure. A planographic printing plate material having such a plate inspection property comprises a colorant, and the colorant is released in a printing press during printing, resulting in contamination of the printing press. That is, the other problem is that a colorant or a color producing agent contained in a planographic printing plate material contaminates prints or a dampening solution.
As planographic printing plate materials requiring no development, there are ones disclosed in Japanese Patent O.P.I. Publication Nos. 2003-25750, 2003-39840 and 2003-246155, in which the image formation layer contains a specific heat fusible polymer, whereby developability and printing durability are improved, and printing press contamination is minimized. However, there is neither disclosure nor suggestion in these documents of a visible image formation that employing a colorant, a visible image is formed after imagewise exposure.
A positive working planographic printing plate material comprising a layer to be rendered hydrophilic by heat and a method of preventing stain occurrence at non-image portions are disclosed in Japanese Patent O.P.I. Publication No. 11-174685. However, there is no disclosure in this document of improvement in printing durability and visible image formation after exposure.
As a conventional technique for forming a visible image without contamination of a printing press and printed matter, there is a technique in which a colorless leuco dye is reacted with a developing agent to form a dye image. In order to secure a practically acceptable visible image, a large amount of the leuco dye is incorporated into an image formation layer, resulting in problem of lowering developability on a printing press. Further, the technique has a problem in that the leuco dye and developing agent in the image formation layer were incorporated into a dampening solution and reacted with each other therein, resulting in contamination of a printing press or printed matter.